The present invention relates generally to refrigeration compressors and more specifically to variable speed refrigeration compressors hving power supplies associated therewith which are designed to vary the frequency of the alternating current supplied to the driving motor whereby the operating speed of the compressor may be selected in accordance with the demand loading on the refrigeration equipment. As used herein the phrase "refrigeration" compressor is intended to include compressors usable to compress a refrigerant, including compressors used in both air conditioning and heat pump applications.
Refrigeration compressors are subject to widely varying demand cycles due to a variety of factors such as for example daily and seasonal fluctuation of ambient temperatures, human activity and presence within the refrigerated area, cyclic operation of other heat generating equipment, etc. Accordingly in order to assure satisfactory refrigeration of the area, refrigeration apparatus including the compressor means must be sized for particular applications to accommodate "worst case" conditions. That is to say that the equipment must have a large enough capacity to assure adequate refrigeration during the worst anticipated condition. Thus, under significantly more favorable operating conditions, which may be present during the major portion of the equipment operating life span, the refrigeration apparatus may have a significant extra capacity and therefore operate at reduced loading most of the time. Because maximum compressor motor operating efficiency is normally obtained only at full motor load, such reduced system load operation may result in less than optimum operating efficiency due to both motor characteristics, relatively short operating cycles and/or frequent starting and stopping of the compressor due to insufficient air circulation within the refrigerated spaces resulting from the shorter operating cycls. Such considerations become even more significant in compressors used for heat pump applications because the system load rating for heating must be up to approximately three times that required for air conditioning. Thus, the actual system operating load varies over a significantly greater range with the air conditioning load being significantly less than that of the heating load. Considering this fact along with the fact that a majority of the actual operation, even for heating, will be under loading conditions less than the full load rating of the equipment, the desirability of providing means by which the capacity of the refrigeration compressor can be modulated in accordance with the demand load can be readily appreciated.
A number of methods have previously been employed in order to modulate compressor capacity such as for example various valving and bypass arrangements which, while effective in modulating the associated system load, also reduce the overall operating efficiency of the refrigeration equipment thus resulting in higher operating costs. This may be particularly significant if refrigeration apparatus is operated at reduced loading for a significant portion of the time such as when heat pumps are being operated in a cooling mode. For example, in one form of compressor modulation, discharge gas is directed from the discharge line directly to the suction line bypassing the condenser and evaporator. Thus, the energy expanded in compression is lost. In another arrangement, particular cylinders may be continuously vented to the intake or suction manifold. While this avoids the expenditure of energy in compressing this gas, it nevertheless still results in reduced overall operating efficiency for a variety of reasons. For example, it is well known that electric motors generally have a maximum operating efficiency when operating at substantially full load. Thus, because the compressor motor must have a full load rating high enough to drive the compressor with all cylinders or compressing chambers full loaded, the reduction in loading due to such cylinder unloading techniques results in a lower motor operating efficiency and hence contributes significantly to lower overall system operating efficiency.
In order to overcome these disadvantages, two speed motors have been used to drive refrigeration compressors. In one form, two speed motors having separate stator windings have been used which include means for selectively energizing one or the other of the two stator windings. However, because of cost and size limitations, only two such windings can economically be provided and thus only two different operating speeds are obtained. In another form, a single stator winding is provided with an increased number of leads being brought out of the motor and connected to switch means adapted to interconnect the stator windings in different groupings, whereby the number of electricl poles produced by the stator winding is changed which results in different motor operating speeds. Typically, however, only two operating speeds may be obtained conveniently which speeds are related by a factor of two (i.e., the lower operating speed will be one-half of the higher operating speed).
While each of these approaches may offer some increase in operating efficiencies over the use of unloading techniques, they are of limited effectiveness because only two distinct operating speeds can be obtained. Unfortunately, demand loading on the refrigeration equipment and particularly heat pumps does not normally shift between two discrete points but rather will vary continuously over a rather broad spectrum. Thus, even the two speed refrigeration compressor motor may spend a significant portion of its operating time at reduced and hence less efficient loading. Therefore, it is desirable to provide a refrigeration compressor having a driving motor which may be operated over a broad range of speeds and also provide a relatively high level of operating efficiency at substantially all of the speeds of operation within the broad range.
The present invention overcomes these disadvantages in providing a refrigeration compressor which includes a two phase motor and an associated power supply which is operable to convert normally available single phase electrical power at a fixed frequency (generally 60 hertz) to two phase power of variable frequency. The use of a two phase motor offers substantially all the same advantages as does the use of a three phase motor over that of a single phase motor such as for example lower locked rotor currents, higher starting torque, lower full load currents, improved reliability due to elimination of the start and/or run capacitors which are required with single phase motors, and reduced stator stack size hence allowing use of smaller housings. The present invention, however, also provides means operative to connect the two phase motor directly to a single phase power source in the event of a failure of the two phase power supply. Thus, the present invention offers improved reliability over that of a three phase motor in that it can be operated directly on single phase power. This is not possible with three phase motors. Further cost savings may be realized both in terms of direct manufacturing and material costs for two phase motors over single or three phase motors and also savings are realized through the ability to use smaller size contactors for connecting to line due to lower locked rotor and running current ratings of the two phase motor. Additionally, costs associated with providing frequency conversion means are less for two phase than for three phase. Further, the present invention incorporates a unique and improved two phase motor design which substantially eliminates the third harmonic by way of selection and positioning of the winding configuration thereby affording substantial improvements in operating efficiencies. Thus, not only does the present invention provide a refrigeration compressor capable of operating at high motor efficiency over a relatively large demand spectrum but further offers improved reliability as well as allowing the advantages of three phase operation to be obtained in locations where only single phase power is available.
Also, in order to insure the two phase power supplied to the motor is of a generally sinusoidal shape, the present invention provides improved high energy filter means which operates to smooth the output waveform of the two phase power supply so as to more closely approximate a true sine wave. In one form, permanent magnet type filter means may be used which requires only relatively few turns of an electrical conductor wound upon a permanent magnet core, although other filter means may also be used.
The present invention also provides improved protection means particularly well suited for use with two phase motors which is equally effective in protecting both phases thereof against excessive current flow as well as affording protection against overheating of the motor. The protection means includes elements connected in series with each of the two phase windings of the motor either of which may operate independently of the other to disconnect the motor should abnormal operating conditions arise in one of the two phases. Additionally, because the protection means is of the line break type, and therefore does not require additional complex external circuitry, it offers relatively high reliability.
Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims taken in conjunction with the accompanying drawings.